1. Field of the Invention
This invention relates generally to a filter arrangement for use in automobile transmission systems and a method of making the filter, and more specifically, this invention relates to a transmission filter having a filter envelope formed by ultrasonically welding thermoplastic fiber felt, the filter envelope being located inside a rigid container.
2. Description of the Prior Art
Current automobile transmission filters are generally formed of two perforated metal plates covered by two sheets of filter material, such as a filtering felt. A metal binder is clamped around the periphery of the filter to hold the metal plates and the sheets of felt in a single structure. This filter structure is submerged in a reservoir of transmission fluid. A suction port leads to the suction line of the transmission system so that the transmission fluid is drawn in through the filtering felt and the perforations in the metal plates. This transmission fluid is then drawn into the suction line from the interior space defined by the metal plates.
This type of structure has some disadvantages. For one thing, as a suction vacuum is created in the interior of the filter the felt is drawn into the perforations of the metal plate to restrict the flow of transmission fluid into the filter. Also, with this type of structure any contaminants are trapped on the outer surface of the filter and are free to re-circulate within the reservoir of transmission fluid.
Another problem with the structure currently used is that as an automobile goes up or down an inclined grade the fluid level in the reservoir is inclined so that a portion of the filter may be exposed to the air, which results in the undesired inclusion of air in the fluid passing into the suction line. In order to prevent the intake of air into the transmission suction line, it is necessary to maintain a relatively large reservoir of transmission fluid, with the attendant cost.
In attempting to solve these problems, the inventors of the current application devised a structure employing a filter envelope located in a rigid or metallic container. While any suitable type of approach could be utilized to form the filter envelope from the layers or sheets of felt, it was decided that ultrasonic welding would be especially desirable to provide the desired sealing connection between the sheets or layers of felt. However, it was discovered that there were a number of difficulties in obtaining the desired ultrasonic weld.
As ultrasonic welding involves a mechanical vibration to produce a friction heat to induce bonding, it means that one layer or sheet of the felt moves relative to the other layer or sheet in the area where it is desired to achieve a weld joint. Damping or limiting the relative motion of one layer or sheet while the other is being ultrasonically vibrated is difficult, in view of the fact that the clamping pressure is being supplied by the ultrasonic horn contacting the non-stationary layer of felt, which tends to produce movement in both layers of felt. Thus, it is very difficult to accurately define the weld area. Also, without the desired amount of damping of the relatively stationary layer or felt, the heat of welding is hard to control and burn-out tends to result. Further, variations in the density or thickness of the felt layers can create serious difficulties in obtaining an appropriate weld.